The present invention relates broadly to a heating apparatus, and in particular to an open element thermal vacuum heater array apparatus.
Thermal vacuum testing of the global positioning system (GPS) vehicle requires specific heater array configurations for the shear panels. The thermal transfer requirements result in four arrays on each side of the vehicle. Each array consists of a metal frame containing Nichrome heater wires with spacing between the wires to provide shear panel cooling, when needed, by radiation to the liquid nitrogen (LN.sub.2) cooled chamber walls.
The array frames and heater wire lengths which are approximately 24 inches in length, operate in a range of service temperatures between -320F. and 1200F. As a result of the wide temperature operating range, the heater wires are subjected to changes in length that result in a compression force in the heater wire if the wire is secure at both ends. The compression load is relieved by central buckling with a lateral deflection of approximately 11/4 inches. The initial distance between the heater wires and spacecraft louvers is three inches and wire to wire spacing two inches. Several fatigue modes exist that can result in a heater wire breaking in prolonged service and nominal test durations are 30 days per global positioning system (GPS) vehicle. Contact of a live heater wire with the spacecraft can be a catastrophic event.
In the prior art, there are two heater array design approaches which have been in use:
1. Arnold Engineering Development Center (AEDC) uses a complex system of ceramic pulleys and spring tension device to ensure the heater wires are not subjected to compression (see FIG. 1). The application is for a single global positioning system (GPS) vehicle on qualification testing.
2. Block I fixtures (see FIG. 2) used heater wires with fixed end terminations. The heater wire movement was controlled by weaving quartz rods and quartz strings through the vertical wires and securing the ends to the side frames of the array. This was used successfully on 11 Block I vehicles. Three cases of heater wire breakage were reported but without an incident. Both of these systems use relatively light gauge wire. The Arnold Engineering Development Center (AEDC) system has two separate wire runs, back to back, in parallel. The Block I system used three twisted strands.
In general, electrical heater panels have utilized multiple numbers of stiff wires in parallel arrays which may be formed in plane or circular configurations. Some prior art heating apparatus have electrical resistance filaments or elements that utilize a heavy Nichrome wire or rod so that they maintain their form when heated. While it is common to secure the heated filament wire to an insulating support, the reverse procedure of placing an insulator upon the heater wire and supporting the insulator in a metal frame is a also common expedient. However, these prior art methods and techniques suffer from the adverse effects of buckling and elongation of the heating elements. Exemplary in the art of electrical resistance heating apparatus are the following U.S. Patents, which are incorporated herein by reference:
U.S. Pat. No. 1,471,913 issued to Otis on Oct. 23, 1923; PA1 U.S. Pat. No. 1,606,765 issued to Keene on Nov. 16, 1926; PA1 U.S. Pat. No. 2,921,172 issued to Hackman on Jan. 12, 1960; PA1 U.S. Pat. No. 3,108,171 issued to Vary on Oct. 22, 1963; PA1 U.S. Pat. No. 3,286,117 issued to Donlevy on Nov. 15, 1966; PA1 U.S. Pat. No. 3,395,241 issued to Roman on July 30, 1968; and, PA1 U.S. Pat. No. 3,798,417 issued to Bittner on Mar. 19, 1974.
The Keene reference, U.S. Pat. No. 1,606,765, discloses an electrical wire heater that uses a relatively heavy Nichrome wire or rod to provide a strong heater wire. The heater wires which are formed into a series-parallel array which are loosely held in position by a plurality of sets of staggered pairs of refractory members of electric-insulating material to hold the intermediate parts of the return-bent straight portions in proper operative positions relative to each other.
The Otis reference, U.S. Pat. No. 1,471,913 discloses a heating unit which utilizes a ribbon of suitable resistance allow which is folded or bent in the shape of a hairpin so as to have two substantially parallel portions which are provided with spacing and supporting insulators of such size that the unit when it is assembled may be slipped freely into the tubes of a fire tube boiler.
The Hackman reference, U.S. Pat. No. 2,921,172 relates to open coil heating units which utilize a long helical coil of resistance wire that is supported in an open or exposed manner by a grille or lattice type metal frame having spaced apart electrical insulators through which the resistance coil extends.
The Vary reference, U.S. Pat. No. 3,108,171 illustrates a radiant heater having formed filaments wherein a plurality of refractory metal strips are arranged in a generally circular array and each filament is curved about its major centerline to form a dihedral angle for structural stability.
The Donlevy reference, U.S. Pat. No. 3,286,117 discloses a filament support which has an elongated bore with a diameter only slightly larger than that of the filament and a length many times the filament diameter.
The Roman reference, U.S. Pat. No. 3,395,241, discloses a heating element assembly comprising a plurality of said heating element rod units interconnected by their bridging elements in a squirrel cage formation with the resistance rods electrically connected in series, and spaced refractory mounts supporting the end portions of the resistance rods.
The Bittner reference, U.S. Pat. No. 3,798,417, describes a heating element assembly with a plurality of elements, with each element being formed by linear reaches or strips of a foil-like resistive conductor immobilized along the length of a supporting rod by adjustable insulating bars at substantially regular intervals. These immobilizing insulating bars prevent axial or rotational movement of the conductive strips of foil to ensure that separate conductors neither short together nor come in contact with the supporting structure or the assembly's containment.